600 B. Chance 



apparently stable for several days (Chance and Spencer, 1959). This result 

 suggests that if conduction bands participate in electron transfer, they are not 

 the sole mechanism of transfer. 



Resonance Energy Transfer 



Ample evidence for resonance energy transfer from the protein to the 

 haera of haemoproteins (Biicher and Kaspers, 1946; 1947) is provided by 

 the quantum requirement of unity for the splitting of myoglobin-carbon 

 monoxide by ultra-violet light. It has also been shown that the haem of the 

 native protein quenches the fluorescence of the free protein (Weber and Teale, 

 1959). In fact, the transfer of the excited state to the haem is found to be 100 

 times more probable than the fluorescence and 20 times more probable than 

 other competing radiationless transfers (Weber and Teale, 1959). In view of 

 these quantitative data, we have again considered the possibility put forward 

 by Keilin and Hartree (1953) and more recently by Dixon and Webb (1958) 

 that anomalies in the photochemical action spectra for the relief of carbon 

 monoxide-inhibited respiration are caused by energy transfer from cyto- 

 chromes other than the oxidase to the haem-CO compound. For example: 



hv 



substrate -^fp— ''Zj— ^Ci-^c-^fl-^Og-^ CO (4) 



Two results suggest that the protein portions of the members of the 

 cytochrome system do not absorb energy and transfer it along the chain to 

 the haem-CO compound. First, as indicated by the above data, the haem 

 of the protein absorbing the light would quench the fluorescence and thus 

 prevent energy transfer to the next member of the chain, unless the possibility 

 of a radiationless transfer were 20 times greater for the carriers embedded in 

 the respiratory chain than for the haem proteins studied by Weber and Teale 

 (1959). 



The latter possibihty is rendered even less likely by the second consideration, 

 which is based on the ratio of the intensities of the Soret band and the 

 protein band (1-85), shown in Warburg's photochemical action spectrum 

 (1949). Such a high ratio would be very unhkely if light absorption in the 

 protein portions of other haemoproteins were also contributing significantly 

 to the spUtting of the haem-CO compound. It should be noted that this ratio 

 (1-85) is appropriate to haemoproteins having a rather higher number of 

 aromatic amino acids and/or a higher molecular weight than cytochrome c, 

 which has a value of 3-7 (KeiUn and Slater, 1953). Currently available 

 preparations (Okunuki, Hagihara, Sekuzu and Horio, 1958) of cytochrome 

 oxidase show a ratio of --^ 0-4 and thus are not yet pure enough to be helpful 

 on this point. 



The possibility that light is absorbed by the haem and is transferred to its 



